This invention relates to the detection of ozone and, in particular, to the detection of slightly higher than normal concentrations of ozone in air at ambient temperatures and pressures.
More specifically, this invention relates to the use of a semiconductor to detect the presence of ozone in air. Gas detectors formed of porous N-type semiconductors have been used for quite some time to detect the presence of reducing agents, such as carbon monoxide or the like. The typical application of this type of device has been in fire warning systems. In operation, a porous sensor adsorbs the reducing agents or gases present in smoke whereby a reduction reaction takes place causing the conductivity of the sensing element to increase and thereby trigger an alarm circuit. This type of semiconductor element, and the method of manufacturing the same, is disclosed in a series of United States patents to Taguchi which include: U.S. Pat. Nos. 3,625,756, 3,631,436, 3,644,795, 3,676,820, 3,695,848, 3,732,519, 3,835,529, 3,900,815.
As disclosed by Taguchi, the semiconductor sensing element is generally formed of a high valance oxide capable of giving up an oxygen molecule to the adsorbed reducing agent. At the same time, two valance electrons are released to the conduction band of the element thus rendering it more conductive. Because of the amount of energy needed to produce the reaction is relatively high, the element is normally provided with a heater adapted to maintain the sensor at an elevated operating temperature. In fact, the high valence oxide semiconductor's sensitivity to a reducing agent, if dicernible at all, at room temperatures is weak and unpredictable.
Accordingly, it has heretofore been thought that oxide type semiconductors were suitable only for detecting the presence of certain reducing agents at elevated temperatures. However, as will be explained in greater detail below, this type of detector can be adapted to sense the presence of slightly higher than normal concentrations of ozone in air at ambient temperatures and pressures. The existence of small amounts of ozone in air is deemed to constitute an environmental hazard particular to those suffering from cardia-vascular ailments.
One area where overexposure to ozone is of primary concern is in the field of aviation where high flying aircraft are capable of remaining in the earth's ozone layer for relatively long periods of time. As the exact location of this layer is constantly in response to many factors, the pilot of an aircraft has no existing way of detecting the danger. As a consequence, cabin air which is drawn from surrounding outside air, can become contaminated with high levels of ozone thereby overexposing the passengers and crew for what could be extended periods of time. Presently, equipment for quickly and rapidly detecting this hazardous situation, while at the same time withstanding the rigors of high altitude flying, is not available.